Fuel cells are receiving increasing attention as a viable energy-alternative. In general, fuel cells convert electrochemical energy into electrical energy in an environmentally clean and efficient manner, typically via an oxygen reduction reaction (ORR). Fuel cells are contemplated as potential energy sources for everything from small electronics to cars and homes. In order to meet different energy requirements, there are a number of different types of fuel cells in existence today, each with varying chemistries, requirements, and uses.
As one example, Direct Methanol Fuel Cells (DMFCs) rely upon the oxidation of methanol on an electrocatalyst layer to form carbon dioxide. Water is consumed at the anode and produced at the cathode. Positive ions (H+) are transported across a proton exchange membrane to the cathode where they react with oxygen to produce water. Electrons can then be transported via an external circuit from anode to cathode providing power to external sources.
As another example, polymer electrolyte membrane (PEM) fuel cells (also called proton exchange membrane fuel cells) use pure hydrogen (typically supplied by a hydrogen tank) as a fuel. A stream of hydrogen is delivered to the anode side of a membrane-electrode assembly (MEA), where it is catalytically split into protons and electrons. As with the DMFC, the positive ions are transported across a proton exchange membrane to the cathode where they react with oxygen to produce water.
Regenerative fuel cells run in reverse mode. For example, as described above, PEM fuel cells use hydrogen and oxygen to produce water and electricity using an ORR catalyst. Run in reverse mode, the fuel cell uses an Oxygen Evolution Reaction (OER) catalyst to use water and electricity to produce hydrogen and water. An external voltage is applied and water at the cathode side of the fuel cell undergoes electrolysis to form hydrogen and oxide ions which are transported through the electrolyte to the anode where they are oxidized to form oxygen. In OER mode, the polarity of the cell is the opposite of that for the ORR mode.
Currently, one of the limiting factors in the wide scale commercialization of both ORR and OER is the cost associated with the precious metals that are used to produce effective catalysts. Both DMFC and PEM fuel cells commonly use platinum as an electrocatalyst, while regenerative fuel cells often use iridium oxide or other rare platinum-family oxides in the coating of anode-electrodes for electrolysis.
Transition metal-oxide systems are known catalysts for both ORR and OER. Previously described methods of forming transition metal-oxide catalysts typically involve pyrolyzing salts of the desirable metals to obtain solid particles of the desired metals, which are then milled to make a powder. It is of particular note that this method will only form solid particles and that porous particles cannot be formed using this methodology. However, previously described transition metal-oxide systems possess a number of disadvantages including low stability in alkaline media, low activity compared to conventional ORR/OER catalysts (i.e., platinum/iridium oxide), low surface area etc.
Accordingly, novel, inexpensive methods of producing stable ORR and OER catalysts with sufficient activity suitable for commercial fuel cell use are desired.